How to assign a custom mac address to addon/container?

Hi everyone!
I am trying to build my first addon, and i need to assign a custom mac address to it. Building the addon, installing in hassio and running works all fine. But docker only has the mac address parameter in the first run phase, when a container is created from an image. There is no option for that during build, so i can’t use build.json for it. And there is nothing for this in config.json
I can’t find where to pass that parameter to docker through hassio. When i create the container via portainer, it works fine, but i need tight control of the container in hassio. Any ideas? Thanks!